In recent years, as one of the causes of global warming, greenhouse effects caused by CO2 are pointed out. From the viewpoint of protecting the earth environment, the problem of greenhouse effects needs to be rapidly solved worldwide. Sources of CO2 are encompassed in any people's fields of activity where fossil fuel is burned, and there have been increasing demands for suppressing the discharge of CO2. For meeting the demands, with respect to power generation plants which use a great amount of fossil fuel, such as thermal power stations, vigorous studies are made on a method in which combustion off-gas of a boiler is brought into contact with, for example, an aqueous solution of alkanolamine to remove and recover CO2 from gas and a method in which the CO2 recovered is stored without being released to the air. An aqueous solution of alkanolamine reacts with not only CO2 (carbon dioxide) but also acidic gas, such as H2S (hydrogen sulfide). Therefore, in chemical industries, for the purpose of removing and recovering general acidic gases, the alkanolamine solution is widely used in refining processes for various gases.
Examples of alkanolamines include monoethanolamine (MEA), ethylaminoethanol (EAE), triethanolamine, N-methyldiethanolamine (MDEA), diisopropanolamine, and diglycolamine, and, generally, monoethanolamine (MEA) is preferably used.
Further, an effect of a mixture of a plurality of alkanolamines such that the mixture exhibits a performance which cannot be achieved by the individual alkanolamine is disclosed (for example, the performance of a mixture of MEA and AMP is disclosed. Patent Document 1). For improving the alkanolamine in an absorption performance, the use of a cyclic amine, such as piperazine, or a linear amine, such as ethylenediamine, as an absorption auxiliary agent has been proposed (Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5).    Patent Document 1: Japanese Patent Application Laid-open No. H6-343858    Patent Document 2: Specification of U.S. Pat. No. 4,336,233    Patent Document 3: Japanese Patent Application Laid-open No. H1-231921    Patent Document 4: Japanese Patent Publication No. 561-19286    Patent Document 5: Specification of U.S. Pat. No. 6,436,174